Heterogeneous Equilibria
A heterogeneous equilibrium involves reactants and products in two or more different phases, as illustrated by the following examples:
| $PbCl_2\; (s)$ | ⇌ | $Pb^{2+}\; (aq) + 2Cl^−\; (aq)$ | $K_c=\frac{[Pb^{2+}]}{[Cl^−]^2}$ |
| $CaO\mathit (s)+CO_2\; (g)$ | ⇌ | $CaCO_3\; (s)$ | $K_c=\frac{1}{[CO_2]}$ |
| $C\mathit (s)+2S\mathit (g)$ | ⇌ | $CS_2\mathit (g)$ | $K_c=\frac{[CS_2]}{[S]^2}$ |
| $Br_2\mathit (l)$ | ⇌ | $Br_2\mathit (g)$ | $K_c=[Br_2\mathit (g) ]$ |
Again, note that concentration terms are only included for gaseous and solute species, as discussed previously.
Two of the above examples include terms for gaseous species only in their equilibrium constants, and so $\mathit{K_p}$ expressions may also be written:
| $CaO\mathit (s)+CO_2\mathit (g)$ | ⇌ | $CaCO_3\mathit (S)$ | $K_P=\frac{1}{P_{CO_2}}$ |
| $C\mathit (s) + 2S\mathit (g)$ | ⇌ | $CS_2\mathit (s)$ | $K_P$=$\frac{P_{CS_2}}{(P_S)^2}$ |
The units of these heterogeneous equilibria are mixed (concentration and pressure); it is important to use only standard units in these calculations ($\frac{mol}{L}$ for concentrations and $bar$ for gas pressures).